1. Field of the Invention
The invention relates to an image forming apparatus such as a copier, a facsimile machine and a printer, and a fixing device suitable for use in such an image forming apparatus. More specifically, the invention relates to a fixing device configured to apply a fixer to resin particles on a fixing medium subject to image fixation or a resin particle carrier subject to image fixation by dissolving or swelling at least a part of the resin particles such as toner, and also to an image forming apparatus including such a fixing device.
2. Description of the Related Art
Image forming apparatuses such as printers, facsimile machines and copiers are generally configured to form images including characters or symbols on fixing media such as paper, cloth, and OHP sheets based on image information. There are various types of image forming apparatuses; however, among these, electrophotographic image forming apparatuses are widely used in office environments because the electrophotographic image forming apparatuses are capable of forming high definition images on plain paper at high speeds. In such electrophotographic image forming apparatuses, a fixing device that employs a thermal fixing system is widely used. In the fixing device employing the thermal fixing system, toner onto a fixing medium such as a recording medium is melted or softened by heating, and the softened toner is then pressed on the recording medium. Accordingly, the toner is fixed on the recording medium. The fixing device employing the thermal fixing system (hereinafter also called a “thermal fixing device”) is suitable for use in the electrophotographic image forming apparatus because it may provide high fixing speeds and high fixing image qualities.
However, more than half of the power is consumed for heating toner in the electrophotographic image forming apparatus having the thermal fixing device. Meanwhile, in view of recent environmental issues, there are increasing demands for low-power consumption (energy conservation) image forming apparatuses. With the low-power consumption image forming apparatus, the power consumed by the fixing device is preferably reduced down to half of the power consumption of the related art image forming apparatuses. Since the related art thermal fixing device is configured to consume a large amount of energy for heating (heating process), there is an increasing demand for a fixing device having a fixing system capable of heating toner to be fixed to the recording medium at an extremely low temperature or capable of fixing toner to the recording medium without heating the toner. In particular, it is ideal to provide a fixing device employing a non-thermal fixing system (hereinafter also called a “non-thermal fixing device”) capable of fixing toner to the recording medium without heating toner in terms of low power consumption. Japanese Patent No. 3290513 (hereinafter referred to as “Patent Document 1”), Japanese Patent No. 4185742 (hereinafter referred to as “Patent Document 2”), Japanese Patent Application Publication No. 59-119364 (hereinafter referred to as “Patent Document 3”), and Japanese Patent Application Publication No. 2009-008967 (hereinafter referred to as “Patent Document 4”) disclose examples of the non-thermal fixing device, that is, a fixing device with a wet fixing system (hereinafter also called a “wet fixing device”). The wet fixing device is configured to apply a fixer containing a softener for softening toner by dissolving or swelling at least a part of a resin component of the toner on a toner image formed on a surface of a recording medium to fix the toner image on the recording medium. In the wet fixing device, since heating is not required for softening the toner, it is possible to save more energy in comparison to the thermal fixing device.
In the configuration of any one of the fixing devices disclosed in Patent Documents 1 to 3, an application roller that is a contact-type fixer application unit is configured to apply a liquid fixer to an unfixed toner image formed on a fixer application target such as a recording medium or an intermediate transfer member to fix the toner image to the fixer application target. In the above configuration, where the toner image is fixed to the recording medium using the liquid fixer, it may be difficult to apply a small amount of the liquid fixer to the toner image on the recording medium while simultaneously preventing the offset of the toner onto the application roller. The difficulty is described in detail below.
If the thickness of a liquid fixer layer applied on the application roller is made less than the thickness of the unfixed toner image in order to apply a small amount of the liquid fixer to the unfixed toner image on the recording medium in the above configuration where the liquid fixer is applied to the unfixed toner image on the recording medium by the application roller, the following difficulties may be involved. Toner particles in the toner layer of the toner image on the recording medium are attracted due to surface tension of an unapplied, remaining liquid fixer film on the application roller at a position where the surface of the application roller is detached from the toner image on the recording medium after the surface of the application roller is brought into contact with the toner image on the recording medium. Accordingly, the offset toner particles on the surface of the application roller may be attached to the toner image on the recording medium when the application roller has been detached from the toner image on the recording medium, thereby degrading the toner image on the recording medium.
If, on the other hand, the liquid fixer on the application roller is made sufficiently thicker than the unfixed toner layer on the recording medium, the surface tension of the liquid fixer film on the application roller may not sufficiently act on the toner particles of the toner layer of the toner image at a position where the surface of the application roller is detached from the toner image on the recording medium after the surface of the application roller is brought into contact with the toner image on the recording medium due to too much of the liquid fixer on the application roller. In this manner, although few toner particles are attached to the surface of the application roller, a large amount of liquid fixer is applied to the toner image on the recording medium. Thus, image quality of the toner image may be degraded due to toner particles flowing in the over-applied liquid fixer dispersed over the toner image on the recording medium, fixing responsiveness may be degraded due to prolonged drying time of the liquid fixer. Further, a user may feel residual liquid (i.e., wet feeling of touch when touching paper with bare hands). Moreover, if the liquid fixer contains water and if a large amount of the liquid fixer is applied to the recording medium such as paper containing cellulose, the recording medium such as paper may be severely curled, which may cause paper jamming during the recording medium transfer inside an apparatus such as an image forming apparatus.
Thus, in the above configuration where the liquid fixer is applied to the toner image on the recording medium by the application roller, image degradation due to toner particle flow, low fixing responsiveness due to prolonged drying time of the liquid fixer, or paper jamming due to cellulose of paper may be obtained. If on the other hand, a small amount of the liquid fixer is applied to the toner image on the recording medium in order to prevent the above case, the toner particles may be transferred and attached to the surface of the application roller as described above. Thus, in the configuration where the liquid fixer is applied to the toner image on the recording medium by the application roller, it may be difficult to apply a small amount of the liquid fixer to improve the fixing responsiveness, lower the residual liquid feeling or prevent the recording medium from curling while simultaneously preventing the toner particles from being transferred to the application roller.
Japanese Patent Application Publication No. 2009-008967 discloses (hereinafter called “Patent Document 4”) another example of the fixing device having the non-thermal fixing system capable of applying a small amount of a fixer to a toner image on a recording medium while simultaneously preventing the transfer (offset) of toner particles onto the application roller. The fixing device disclosed in Patent Document 4 further includes a foam fixer forming device configured to generate a foam fixer by dispersing air voids in a liquid fixer and the generated foam fixer is applied on the toner image formed on a recording medium. With this configuration, since the fixer is foam, particle (toner) density of the fixer may be reduced. Thus with this non-thermal fixing system, the film thickness of the fixer on an application roller may be increased with an amount of the fixer less than that of the related art liquid fixer. As a result, it may be possible to reduce an adverse effect of liquid surface tension on the toner particles on the recording medium. Further, since the amount of the fixer applied to the toner image on the recording medium is small, residual liquid feeling of the recording medium may be reduced. Further, since the foam fixer does not easily flow compared to the liquid fixer, image degradation due to toner particles flowing in the fixer may also prevented. Thus, the toner image may be fixed with the amount of the fixer less than that of the related art fixer without degradation if the toner image is fixed with the foam fixer as described in Patent Document 4.
The fixer device disclosed in Patent Document 4 includes a pressure roller configured to be brought into contact with the application roller to form an application nip, through which the recording medium is passed while the foam fixer on the application roller is applied onto the toner image on the recording medium. However, in the above fixing system where the foam fixer is applied by the application roller, the foam fixer may accumulate around an entrance of the application nip.
If the recording medium carrying an unfixed toner image on its surface is transferred to the entrance of the application nip where the foam fixer is accumulated, the recording medium is brought into contact with the accumulated foam fixer before being inserted into the application nip. When the recording medium is brought into contact with the accumulated foam fixer, a portion of a surface layer of the unfixed toner image on the recording medium is dissolved in the accumulated foam fixer and the portion dissolved into the accumulated foam fixer is detached from the toner image on the recording medium and transferred to the application roller. Further, in the configuration where the foam fixer is applied, the amount of the foam fixer carried on the application roller is adjusted such that a small amount of the liquid fixer is applied to the toner image on the recording medium while simultaneously preventing toner from being transferred (offset) to the application roller. However, if the foam fixer is accumulated around the entrance of the application nip, too much of the foam fixer may be applied to the toner image on the recording medium, thereby degrading image quality caused by image flowage due to applying too much of the foam fixer. Further, if the accumulated foam fixer (i.e., foam accumulation) is increased around the entrance of the application nip, the accumulated foam fixer may prevent the recording medium from being inserted (transferred) into the application nip, which may cause recording media jamming.
Note that an application member used as a contact-type fixer application unit configured to apply the foam fixer to a fixer application medium is not limited to the application roller and may be a belt type member. Note that an application member used as a contact-type fixer application unit configured to apply the foam fixer to a fixer application medium is not limited to the application roller and may be a belt type member. The foam fixer accumulation around the entrance of the application nip may occur in any configuration where the foam fixer is applied to the toner image on the recording medium at the application nip where the surface moving application member is brought into contact with another member. Further, as an example of the fixing device to fix the toner image to the recording medium using the foam fixer, there is provided a fixing device configured to apply the foam fixer to a toner image carrier member such as an intermediate transfer belt configured to carry the toner so as to simultaneously transfer the toner image and the foam fixer applied to the toner image on the toner image carrier member to the toner carrier member. As described above, even if the fixing device applies the foam fixer not to the toner image on the recording medium but to the toner image on the toner image carrier member, image degradation due to the foam fixer accumulated around the application nip may occur insofar as the configuration of the fixing device forms the application nip. Note that the image degradation due to the foam fixer accumulated around the application nip may occur insofar as the foam fixer is applied to any resin-containing particles to fix the toner image to the recording medium.